1. Technical Field of the Invention
The present inventions relate to circuit boards, more particularly, to impedance compensation for circuit board breakout regions.
2. Background Art
Circuit boards, such as printed circuit boards (PCBs), have chips (integrated circuits or die) on one or both sides of the circuit board. The chips are typically protected by a package. A variety of connection approaches may be used to make connections between the PCB, package, and die. The interface of the PCB with the package often has a circuit board breakout region where the traces from package balls are more densely populated for a small length prior to fanning out on the PCB. This may be used to interface the larger PCB physical features with the smaller package features.
A typical prior art approach is illustrated schematically in FIG. 1. Referring to FIG. 1, a system 10 includes a packaged chip 14 supported by a PCB 16. Packaged chip 14 includes a die 22 and an associated package 20. Support 24 is a portion or portions of package 20 that supports traces. For ease of illustration, die 22 is shown on an edge of package 20, but that is not required in actual use. Die 22 has die bumps 26 of which die bumps 26A, 26B, 26C, 26D, 26E, and 26F are illustrated. Die bumps 26 are an interface between package traces 30 and internal circuitry of the die. Package 20 has package balls of which package balls 36A, 36B, 36C, 36D, 36E, 36F, 36G, and 36H are illustrated. Package traces 30 couple corresponding die bumps 36 and package balls 26. For example, package trace 30A couples package ball 36A to die bump 26A, package trace 30B couples package ball 36B to die bump 26B, etc.
PCB traces 40 are positioned on PCB 16. Examples of PCB traces 40 include traces 40A, 40B, 40C, 40D, 40E, and 40F. PCB traces 40 from PCB 16 couple to corresponding package balls 36. For example, PCB trace 40A couples to package ball 36A, PCB trace 40B couples to package ball 36B, etc. PCB 16 includes multiple layers and different ones of traces 40 may be in different layers of or on the same layer of PCB 16 and can change layers through vias.
A PCB breakout region 38 is a region in which PCB traces 40 connect to package balls 36. There is a relatively small amount of space for traces 40 to connect to package balls 36 when compared with the amount of space on PCB 16 for traces 40 to be positioned. Generally, there is a higher density of traces 40 in breakout region 38 than outside breakout region 38. Density refers to the number of portions of traces that are included in a unit area. With a higher density of traces, there may be a greater danger of problems such as cross-talk. One way to allow the higher density in breakout region 38 while reducing cross-talk or similar problems is to have traces 40 be narrower inside breakout region 38 than outside. For example, section 40A2 of trace 40A is narrower than is section 40A1 of trace 40. Traces 40 are illustrated as fanning out at different angles, but this is shown to illustrate a lower trace density, not to show an actual fan-out. The actual position of traces in the fan-out may be different than that illustrated.
The impedances Zo of the narrower portions of traces 40 in breakout region 38 (e.g., trace section 40A2) are greater than those wider portions outside breakout region 38 (e.g., trace section 40A1). This presents an impedance discontinuity to signals propagated through this region, which is not desired. Impedance discontinuities can create unwanted reflections, thereby slowing the speed at which signals can be reliably switched. Merely as an example, the impedances of traces 40 may be 50 ohms outside breakout region 38, but increase to 60 ohms where PCB traces narrow inside breakout region 38. Package traces 30 may be 50 ohms again.
A package breakout region 32 refers to a region in which package traces 30 connect to die bumps 26. The density of package traces 30 may be generally lower between PCB breakout region 38 and package breakout region 32 than in either PCB breakout region 38 or package breakout region 32. Although package traces 30 are narrower than PCB traces 40, the impedance of package traces 30 may be the same as PCB traces 40 where other dimensions such as the distance between the trace and ground plane are also scaled.
The impact of an impedance discontinuity increases as the frequencies of signals on the traces increases.